In medicine, there are many situations where it is necessary to aid the human body to build new bone. Physical trauma, such as fractures, may damage the bone tissue in such complex ways, which makes standard bone fracture treatments insufficient. Tumours, which destroy large portions of bone tissue, can make it impossible for the body to heal the injury by itself. Another example is so called sinus lifts, where the natural bone is too thin to support a dental implant and the bone tissue has to be augmented.
The insertion of a bone scaffold is a common way of solving these issues. Bone scaffold materials contain a structure and composition, which will trigger the formation of bone when implanted in the human body, and act as a cultivating substrate for bone cells. Some bone scaffolds are also able to withstand the mechanical loads, which was excerted on the original bone tissue. Natural bone consists of rod-shaped calcium phosphate crystals with a length of 20-40 nm, 2 nm thick and 2-4 nm wide, surrounded by a collagen network1. HA, with the chemical formula Ca5(PO4)3OH, is a mineral, which closely resembles the calcium phosphate mineral found in natural bone.
Synthetic bone scaffolds can be in the form of a powder, such as HA or tricalcium phosphate. Powder based products are successful in restoring bone tissue, but are less suitable for load-bearing applications. There are also injectable, hardening bone scaffold materials, which aim to ease the insertion procedure. One example is a mixture of HA and calcium sulfate hemihydrate, this composition is mixed with water and injected into the desired area. The calcium sulfate reacts with water and serves as a “glue” between the HA crystals. The calcium sulfate crystals are subsequently resorbed by the body, resulting in a porous structure, which in turn creates a suitable environment for bone cell growth2. Another type of bone scaffold is the rigid type, these usually consist of porous HA or tricalcium phosphate, which has been sintered at high temperatures. Some commercial products use natural coral as a raw material. The coral is reacted with phosphate salts at high temperature, and the resulting structure is a rigid porous HA material3. Due to environmental concerns, synthetic methods to produce the porous HA structure are becoming more common. The primary advantages of rigid bone scaffold products are the non-shrinking structure of the material, and the porosity, which increases the available surface area for the bone cells to grow on. One disadvantage is the brittleness of the structure, due to the fact that the material contains no substances, such as polymers, which may dissipate forces applied to the material, as in natural bone. Another disadvantage is that the material requires careful fitting before it can be inserted in the body.
One way of decreasing the brittle nature of a rigid mineral scaffold is to incorporate a polymer into the structure. Polymer/mineral composite materials consist of a polymer, which can be biodegradable, and a mineral with bone regeneration properties, such as HA. The polymer makes the composite elastic and crack-resistant, while the mineral induces the formation of endogenous bone. The biodegradation speed can be governed by the choice of polymer. Biodegradable polymers include synthetic polyesters such as poly(caprolactone) (PCL), poly(lactide) or poly(lactic acid) (PLA) and poly(glycolide) (PGL), but also naturally occurring polymers, such as collagen, hyaluronic acid, chitin and chitosan. These polymers undergo hydrolysis in the human body, producing non-toxic degradation products4. PCL is a polyester with the composition (C6H10O2)n. It has a Tg of −60° C. and a melting temperature of 60° C. For biomaterial applications, it is used in sutures, root canal fillings and drug delivery applications. PCL exhibits a significant high degree of elongation until breakage (>700%), which makes it suitable in load-bearing applications4. Melting of PCL produces a paste with a viscosity, which increases with the molecular weight. A PCL melt with a molecular weight of 80000 g/mol is a viscous, sticky substance. Above the melting temperature, it can be casted to any desired shape. Upon cooling, the polymer rapidly becomes more viscous, since the polymer chains aggregate and become less mobile. Composites of PCL/HA are reported in the literature5,6 and have been shown to have good mechanical properties and also to induce bone cell growth. However, these composites are generally too stiff to be shaped at room temperature.
For making dense, non-porous polymer/mineral composites, the mixing is generally undertaken by melt extrusion7,8 or solvent/solution casting8,9. Dense composites have high strengths but are lacking in osseointegration properties since the bone cells have less surface area to grow on. A porous structure is a better scaffold for bone cell growth. One method of making a porous structure is to add a so called porogen, i.e. a material, which supports the initial structure and is subsequently removed by washing or heating10. A common porogen is sodium chloride, which is readily removed with water11-13 (U.S. Pat. No. 5,766,618) before the implantation.
In addition to the porosity of the bone scaffold, the crystal size of the calcium phosphate is also important to stimulate the growth of endogenous bone. For certain biomaterial applications, it is highly desirable to use nanosized HA, i.e. with a particle size of 1-100 nm in length. Other terms for particles in this size interval may be “nanocrystalline” or simply “nanoparticles”. It is generally considered that the bioactivity of HA is improved if the HA crystals are of a similar size and shape as those produced by the human body. The body recognizes the nanosized HA as a part of its own bone tissue and starts to grow new bone around the foreign object. For implants, a coating with nanosized HA will significantly increase the bone cell activity compared to microsized HA14,15. For polymer/HA composites, the bioactivity as well as the strength is greatly improved with nanosized HA16,17.
For many situations it is highly advantageous if the polymer/HA composite is shapeable or injectable at room temperature, or at temperatures that are close to the human body temperature. When implanted, and after hardening, the material should be able to withstand high mechanical loads, and preferably the material should be porous to enable the ingrowth of bone cells. The composite should also contain HA particles of the same size and shape as those found in the human body in order to stimulate the growth of endogenous bone.
In the literature, several patents describe products, which aim to solve the above needs. WO2007015208A describes an injectable bone scaffold comprising poly(vinyl alcohol), water and tricalcium phosphate, which upon mixing generates a hydrogel. Depending on the amount of polymer present, the composite can be readily injected in cavities in the time range of 2-60 minutes. The hardening is induced by leakage of water into the surrounding media. However, unlike the invention herein, this patent application employs a polymer, which is degraded very slowly in the human body. Furthermore, the composite comprises a non-porous and dense bone scaffold.
U.S. Pat. No. 6,331,312 describes a method of producing a bone scaffold material, consisting of poorly crystalline apatite together with biodegradable polymers. The product is mixed with water, which creates a mouldable composite. However, unlike the invention herein, this patent application describes preparation routes mainly intended for attaining non-porous and dense composites.
U.S. Pat. No. 7,004,974 describes a substance, which consists of calcium phosphate granules, lipid and hyaluronic acid. When mixed with water, this substance generates a mouldable and injectable composite, with relatively low compression strengths.
US2006013857 describes different compositions, which have the form of a putty at body temperature and which are hard at room temperature. The compositions contain gelatin, calcium stearate, tocopheryl acetate and in some examples microsized HA particles (6-12 μm). This document does not describe the use of nanosized HA, nor does it describe a method to control the hardening speed of the composite.
There are a few documents on the use of biodegradable polymers together with HA and plasticizers. U.S. Pat. No. 7,186,759 for example, describes a three component system consisting of a biocompatible polymer, a water-soluble or hydrolytically degrading polymer, such as poly(ethylene glycol) and a bioactive substance. The composite can be softened upon heating and hardened upon cooling. The bioactive substance may be a substance, which can induce bone growth, such as hydroxyapatite. Upon removal of the hydrolytically degradable component, for instance upon contact with water or other fluids present in the human body, a porous structure will be generated with the bioactive substance present in the pores, in the polymer matrix or at the outer surface of the composite material. However, even though U.S. Pat. No. 7,186,759 discloses a composite that contains a porogen in the form of a water-soluble or hydrolytically degrading polymer, the mouldability has proven to be restricted to a short period of time. It should also be noted that the degradation speed of PEG is in the same range as the degradation of the supporting polymer18,19 and the desired porosity of the composite upon removal of the PEG polymer will not be very efficient. Furthermore, the patent employs micrometer sized HA particles.
SE520688 reports on an injectable bone replacement material, which is composed of two parts. One part contains a biologically active substance, such as a biologically compatible oil. The second part comprises bone cement consisting of calcium sulphate (in order to accelerate the hardening process) and/or a bone mineral substitute, such as HA (in the size range of 10 μm, preferably smaller). Mixing the two components renders a bone replacement material, which is of low viscosity, enabling facile injection of the material into the area of choice. The material can either be injected to fill the void between an implant and the surrounding tissue or as a sole component. The bone replacement material can also be moulded into various shapes before being inserted into the body since the maximum hardness is reached after approximately 4 to 8 minutes depending on the composition. However, the invention does not employ biodegradable polymers as primary components.
WO2008/000488 describes a biomaterial for tissue regeneration, which may consist of a bioactive material, such as beta-tricalcium phosphate, a biodegradable polymer, such as poly(lactic-co-glycolic acid), and a water binding agent, such as calcium sulfate, to decrease the degradation of the biodegradable polymer. This document also describes the use of a compound, such as poly(ethylene glycol) 400, to improve the dissolution of the biodegradable polymer. This document does not describe the use of a plasticizer to prolong the shapeability of the composite, and does not describe the use of nanosized HA.
As previously mentioned, it is highly advantageous for a bone substitute to 1) be shapeable and injectable at room temperature or at a temperature not exceeding 37° C. for a long period of time, 2) be able to withstand high mechanical loads, 3) be porous to enable the ingrowth of bone cells and 4) contain HA particles of the same size and shape as those found in the human body in order to stimulate the growth of bone cells as efficiently as possible.
The method of combining the above mentioned approaches either simultaneously or in sequence for synthesizing a strong, mouldable composite, has not been previously disclosed.